Recently, with the flood of information provided by various media such as broadcasting and press, an atmosphere has been created by the information providers who are interested in providing integrated information that covers all of the media. Other users want to selectively receive a specific item of digital information from the entire spectrum of information available from a particular information provider (IP). Accordingly, a digital content transmission system has been formed by the information providers who convert various types of information into a digital form and store this digital information, and the users who subscribe to this digital information system from the information provider via the network. Digital information transmission systems endow an application program with easy downloadability of the digital content. The user can get all the information desired by using this application program to access the digital information system through the network.
The digital information may be provided to the user either for pay or for free. In case of paid digital information, the server who provides the digital information via the transmission system sets the service fee. The service server charges the user according to the quantity of information used when the digital information is downloaded to the user. MPEG software protocol for example, compresses audio files to a fraction of their original size, but has little perceptible effect upon the quality of the audio sound. MPEG software protocol is now widely used by Internet sites offering digitalized music, and is reported to be commonly used to offer digitalized versions of recorded music without the consent of the musicians. When a user is connected to a server that provides digital information commercially via a network, a few of the users may be able to inadvertently or illegally copy the digital information, a practice that, as was recently noted by Interdeposit and the French Agency for the Protection of Programs, a member of the European Association of Authors and Information Technology Professional, in the Patent, Trademark & Copyright Journal, volume 57, No. 1416, page 385 (11 Mar. 1999), would be economically damaging to both the musicians and to the server who is running the digital information transmission system. Currently, the server, as well as the musicians, can do little more than seek redress by undertaking civil and criminal action in an effort to control the possibility of unlicensed reception of digital information. We have noticed that there is a need for a technique to preserve transmission security of revenue bearing information while restricting access to the information by unauthorized entities and preventing unauthorized users from using any of the information that they may be able to illicitly obtain from the information provider by restricting the ability of the unauthorized users to decrypting whatever information they manage to obtain via the system.
Also, it is difficult to prevent the illegal copy of the supplied digital contents or the CODEC recorded on the portable medium if the portable medium is copied after the digital content has been supplied to a user and recorded on the portable medium.
In particular, the MP3 which is the audio data of the above digital contents is downloaded to the first content output unit as well as the second content output unit such as an MP3 player and then reproduced. In the meantime, the MP3 is downloaded to a content storage unit such as a smartmedia card built in the first content output unit, and the MP3 downloaded in the content storage unit is reproduced through the second content output unit.
However, as stated above, there is a drawback in that the digital data downloaded to the first and second content output units and the content storage unit are easily copied to be illegally distributed.